The invention relates to a rewritable optical information medium for high-speed recording by means of a laser-light beam, said medium comprising a substrate carrying a stack of layers, which stack comprises, in this order, a first dielectric layer, a recording layer of a phase-change material comprising an alloy consisting of Ge, Sb and Te, a second dielectric layer and a metal mirror layer.
The invention also relates to the use of such an optical recording medium in high storage density and high data rate applications.
Optical information or data storage based on the phase change principle is attractive, because it combines the possibilities of direct overwrite (DOW) and high storage density with easy compatibility with read-only systems. Phase-change optical recording involves the formation of submicrometer-sized amorphous recording marks in a thin crystalline film using a focused laser-light beam. During recording information, the medium is moved with respect to the focused laser-light beam which is modulated in accordance with the information to be recorded. Due to this, quenching takes place in the phase-change recording layer and causes the formation of amorphous information bits in the exposed areas of the recording layer which remains crystalline in the unexposed areas. Erasure of written amorphous marks is realized by recrystallizing through heating with the same laser. The amorphous marks represent the data bits, which can be reproduced via the substrate by a low-power focused laser-light beam. Reflection differences of the amorphous marks with respect to the crystalline recording layer bring about a modulated laser-light beam which is subsequently converted by a detector into a modulated photocurrent in accordance with the coded, recorded digital information.
One of the main problems in high speed phase-change optical recording is the required erasing (recrystallization) speed. A high crystallization speed is particularly required in high-density recording and high data rate applications, such as disc-shaped DVD-RAM and optical tape, where the complete crystallization time (complete erase time: CET) has to be shorter than 50 ns. If the crystallization speed is not high enough to match the linear velocity of the medium relative to the laser-light beam, the old data (amorphous marks) from the previous recording cannot be completely removed (recrystallized) during DOW. This will cause a high noise level.
An optical information medium of the type mentioned in the opening paragraph is known from U.S. Pat. No. 5,191,565. The known medium of the phase-change type comprises a disc-shaped substrate carrying a stack of layers consisting, in succession, of a first dielectric layer, a recording layer of a phase-change Ge--Sb--Te alloy, a second dielectric layer and a metal reflective layer. Such a stack of layers can be referred to as an IPIM-structure, wherein M represents a reflective or mirror layer, I represents a dielectric layer and P represents a phase-change recording layer. Said patent discloses in the ternary composition diagram (FIG. 5) a locus of 50 ns pulse times around the stoichiometric compound GeSb.sub.2 Te.sub.4, at which pulse time the Ge--Sb--Te compositions begin to crystallize. This time is not equal to the complete erase time CET, but shorter. The complete erase time CET is defined as the minimum duration of the erasing pulse for complete crystallization of a written amorphous mark in a crystalline environment, which is measured statically. For complete erasure of an amorphous mark, two processes are necessary, i.e. nucleation and grain (crystallite) growth. The time mentioned in said patent is the nucleation time, i.e. the time that the first crystallites can be observed. Complete erasure, i.e. complete crystallization of the amorphous mark, takes some additional ten or more nanoseconds. Said patent teaches that compositions on the GeTe--Sb.sub.2 Te.sub.3 tie-line in the ternary diagram crystallize more quickly. E.g. the stoichiometric compound GeSb.sub.2 Te.sub.4 (Ge.sub.14.3 Sb.sub.28.6 Te.sub.57.1, in atomic percentages) is indicated to have a nucleation time of 40 ns. Experiments by the current Applicant show that this compound has a CET-value of 53 ns.